Frameworks

Six distinct aspects of Tuscany we therefore recognize, two when it was fluid, two when level and dry, two when it was broken; and as I prove this fact concerning Tuscany by inference from many places examined by me, so do I affirm it with reference to the entire earth, from the descriptions of different places contributed by different writers.

Nicolaus Steno (1669) The Prodromus of Nicolaus Steno's Dissertation Concerning a Solid Body Enclosed by Process of Nature Within a Solid

Before the distributions of fossils in time and space can be described, analyzed and interpreted, fossil animals and plants must be described in their stratigraphic context. A rock stratigraphy is the essential framework that geologists and particularly paleontologists use to accurately locate fossil collections in both temporal and spatial frameworks. It seems, not surprisingly, that like a fine bottle of Italian wine, this can be traced back to the sunny, pastel landscapes of Tuscany and the Renaissance.

Leonardo's legacy_

The origin of modern stratigraphy can be traced back to Leonardo da Vinci and his drawings. Pioneer work by the Danish polymath Nicolaus Steno (Niels Stensen) in northern Italy, during the late 17th century (see p. 11), established the simple fact that older rocks are overlain by younger rocks if the sequence has not been inverted (Fig. 2.1a). His law of superposition of strata is fundamental to all stratigraphic studies. In addition, Steno established in experiments that sediments are deposited horizontally and rock units can be traced laterally, often for considerable distances; remarkably simple concepts to us now, but earth shattering at the time. But what has this got to do with da Vinci?

Leonardo da Vinci (1452-1519) is famous for many things, and his contributions to science are refreshingly modern when we look back at them. In his art, da Vinci essentially rediscovered geological perspective, some 200 years before Steno, during the Renaissance (Rosenberg 2001). In his drawing of the hills of Tuscany, da Vinci portrayed a clear sequence of laterally-continuous, horizontal strata displaying the concept of superposition. Moreover, about a century after Steno, Giovanni Arduino recognized, again using superposition, three basically different rocks suites in the Italian part of the Alpine belt. A crystalline basement of older rocks, deformed during the Late Paleozoic Variscan orogeny, was overlain unconformably by mainly Mesozoic limestones deformed later during the Alpine orogeny; these in turn were overlain uncon-formably by poorly consolidated clastic rocks, mainly conglomerates. These three units constituted his primary, secondary and tertiary systems; the last term has been retained and formalized for the period of geological time

Figure 2.1 (a) Steno's series of diagrams illustrating the deposition of strata, their erosion and subsequent collapse (25, 24 and 23) followed by deposition of further successions (22, 21 and 20). These diagrams demonstrate not only superposition but also the concept of unconformity. (b) Giovanni Arduino's primary, secondary and tertiary systems, first described from the Apennines of northern Italy in 1760. These divisions were built on the basis of Steno's Law of Superposition of Strata. (c) Idealized sketch of William Smith's geological traverse from London to Wales; this traverse formed the template for the first geological map of England and Wales. Data assembled during this horse-back survey were instrumental in the formulation of the Law of Correlation by Fossils. (a, from Steno 1669; c, based on Sheppard, T. 1917. Proc. Yorks. Geol. Soc. 19.)

Figure 2.1 (a) Steno's series of diagrams illustrating the deposition of strata, their erosion and subsequent collapse (25, 24 and 23) followed by deposition of further successions (22, 21 and 20). These diagrams demonstrate not only superposition but also the concept of unconformity. (b) Giovanni Arduino's primary, secondary and tertiary systems, first described from the Apennines of northern Italy in 1760. These divisions were built on the basis of Steno's Law of Superposition of Strata. (c) Idealized sketch of William Smith's geological traverse from London to Wales; this traverse formed the template for the first geological map of England and Wales. Data assembled during this horse-back survey were instrumental in the formulation of the Law of Correlation by Fossils. (a, from Steno 1669; c, based on Sheppard, T. 1917. Proc. Yorks. Geol. Soc. 19.)

succeeding the Cretaceous (Fig. 2.1b). These three divisions were used widely to describe rock successions elsewhere in Europe showing the same patterns, but these three systems were not necessarily the time correlatives of the type succession in the Apennines.

There is now a range of different types of stratigraphies based on, for example, lithol-ogy (lithostratigraphy), fossils (biostratigra-phy), tectonic units, such as thrust sheets (tectonostratigraphy), magnetic polarity (magnetostratigraphy), chemical composi tions (chemostratigraphy), discontinuities (allostratigraphy), seismic data (seismic stratigraphy) and depositional trends (cyclo- and sequence stratigraphies). The first two have most application in paleontological studies, although sequence and cyclostratigraphic frameworks are now providing greater insights into the climatic and environmental settings of fossil assemblages. Here, however, we concentrate on lithostratigraphy (rock framework), biostratigraphy (ranges of fossils) and chronostratigraphy (time dimension).

ON THE GROUND: LITHOSTRATIGRAPHY

All aspects of stratigraphy start from the rocks themselves. Their order and succession, or lithostratigraphy, are the building blocks for any study of biological and geological change through time. Basic stratigraphic data are first assembled and mapped through the definition of a lithostratigraphic scheme at a local and regional level. Lithostratigraphic units are recognized on the basis of rock type. The formation, a rock unit that can be mapped and recognized across country, irrespective of thickness, is the basic lithostratigraphic category. A formation may comprise one or several related lithologies, different from units above and below, and usually given a local geographic term. A member is a more local litho-logic development, usually part of a formation, whereas a succession of contiguous formations, with some common characteristics is often defined as a group; groups themselves may comprise a supergroup. All stratigraphic units must be defined at a reference or type section in a specified area. Unfortunately, the entire thickness of many lithostratigraphic units is rarely exposed; instead of defining the whole formation, the bases of units are defined routinely in basal stratotype sections at a type locality and the entire succession is then pieced together later. These sections, like yardsticks or the holotypes of fossils (see p. 118), act as the definitive section for the respective strati-graphic units. These are defined within a rock succession at a specific horizon, where there is a lithologic boundary between the two units; the precise boundary is marked on a stratigraphic log. Since the base of the succeeding unit defines the top of the underlying unit, only basal stratotypes need ever be defined.

A stratigraphy, illustrated on a map and in measured sections, is required to monitor biological and geological changes through time and thus underpins the whole basis of Earth history. It is a simple but effective procedure. Successions of rock are often divided by gaps or unconformities. These surfaces separate an older part of the succession that may have been folded and uplifted before the younger part was deposited. Commonly there is a marked difference between the attitudes of the older and younger parts of the succession; but sometimes both parts appear conformable and only after investigation of their fossil content, is it clear that the surface represents a large gap in time.

Early geologists thought the Earth was very young, but the Scottish scientist James Hutton (1726-1797) noted the great cyclic process of mountain uplift, followed by erosion, sediment transport by rivers, deposition in the sea, and then uplift again, and argued that such processes had been going on all through Earth's history. He wrote in his Theory of the Earth (1795) that his understanding of geological time gave "no vestige of a beginning, - no prospect of an end". An example of Hutton's evidence is the spectacular unconformity at Siccar Point, Berwickshire, southern Scotland, where near-horizontal Old Red Sandstone (Devonian) strata overlie steeply-dipping Silurian greywackes. Beneath the unconformity, Hutton recognized the "ruins of an earlier world", establishing the immensity of geological time. This paved the way for our present concept of the Earth as a dynamic and changing system, a forerunner to the current Gaia hypothesis, which describes the Earth as a living organism in equilibrium with its biosphere. Although the Earth is not actually a living organism, this concept now forms the basis for Earth system science.

USE OF FOSSILS: DISCOVERY OF BIOSTRATIGRAPHY

Our understanding of the role of fossils in stratigraphy can be traced back to the work of William Smith in Britain and Georges Cuvier and Alexandre Brongniart in France. William Smith (1769-1839), in the course of his work as a canal engineer in England, realized that different rocks units were character ized by distinctive groups or assemblages of fossils. In a traverse from Wales to London, Smith encountered successively younger groups of rocks, and he documented the change from the trilobite-dominated assemblages of the Lower Paleozoic of Wales through Upper Paleozoic sequences with corals and thick Mesozoic successions with ammonites; finally he reached the molluskan faunas of the Tertiary strata of the London Basin (Fig. 2.1c). In France, a little later, the noted anatomist Georges Cuvier (see p. 12) together with Alexandre Brongniart (17701849), a leading mollusk expert of the time, ordered and correlated Tertiary strata in the Paris Basin using series of mainly terrestrial vertebrate faunas, occurring in sequences separated by supposed biological catastrophes.

These early studies set the scene for biostratigraphic correlation. In very broad terms, the marine Paleozoic is dominated by bra-chiopods, trilobites and graptolites, whereas the Mesozoic assemblages have ammonites, belemnites, marine reptiles and dinosaurs as important components, and the Cenozoic is dominated by mammals and molluskan groups, such as the bivalves and the gastropods. This concept was later expanded by John Phillips (1800-1874), who formally defined the three great eras, Paleozoic (" ancient life"), Mesozoic ("middle life") and Cenozoic ("recent life"), based on their contrasting fossils, each apparently separated by an extinction event. Many more precise biotic changes can, however, be tracked at the species and subspecies levels through morphological changes along phylogenetic lineages. Very accurate correlation is now possible using a wide variety of fossil organisms (see below).

Biostratigraphy: the means of correlation_

Biostratigraphy is the establishment of fossil-based successions and their use in stratigraphic correlation. Measurements of the stratigraphic ranges of fossils, or assemblages of fossils, form the basis for the definition of biozones, the main operational units of a biostratigra-phy. But the use of such zone fossils is not without problems. Critics have argued that there can be difficulties with the identifications of some organisms flagged as zone fossils; and, moreover, it may be impossible to determine the entire global range of a fossil or a fossil assemblage, so long as fossils can be reworked into younger strata by erosion and redeposition, but this is relatively rare. Nonetheless, to date, the use of fossils in bio-stratigraphy is still the best and usually the most accurate routine means of correlating and establishing the relative ages of strata. In order to correlate strata, fossils are normally organized into assemblage or range zones.

There are several types of range zone (Fig. 2.2); some are used more often than others. The concept of the range zone is based on the work of Albert Oppel (1831-1865). Oppel characterized successive lithologic units by unique associations of species; his zones were based on the consistent and exclusive occurrence of mainly ammonite species through Jurassic sections across Europe, where he recognized 33 zones in comparison with the 60 or so known today. His zonal scheme could be meshed with Alcide d'Orbigny's (18021857) stage classification of the system, based on local sections with geographic terms, further developed by Friedrich Quenstedt (1809-1889). Although William Smith had recognized the significance of fossils almost 50 years previously, Oppel established a modern and rigorous methodology that now underpins much of modern biostratigraphy.

The known range of a zone fossil (Box 2.1) is the time between its first and last appearances in a specific rock section, or first appearance datum (FAD) and last appearance datum (LAD). Clearly, it is unlikely that the entire global vertical range of the zone fossil is represented in any one section; nevertheless it is, in most cases, a workable approximation. This range, measured against the lithostratig-raphy, is termed a biozone. It is the basic biostratigraphic unit, analogous to the lithostratigraphic formation. It too can be defined with reference to precise occurrences in the rock, and is defined again on the basis of a stratotype or basal stratotype section in a type area. Once biozones have been established, quantitative techniques may be used to understand the relationships between rock thickness and time, and to make links from locality to locality (Box 2.2).

This is all very well, of course, but the fossil record is rarely complete; only a small percentage of potential fossils are ever preserved. Stratigraphic ranges can also be influenced by the Signor-Lipps effect (Signor & Lipps 1982),